1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An image forming apparatus typically forms a toner image on an image bearer on the basis of image information, transfers the toner image onto a recording medium such as paper and an OHP sheet, and causes the recording medium that bears thereon the toner image to pass through a fixing unit, so that the toner image can be fixed onto the recording medium through heat and pressure.
The transfer unit as transfer means that transfer the toner image can, however, cause an unusual image to occur. Such an unusual image can occur due to, for example, an electric discharge occurring as a result of an output of an abnormal voltage with an increased load that results from, for example, deterioration of a transfer roller over time.
In contrast, the system applies a current for estimation before paper feeding is started to detect a current-voltage characteristic, thereby controlling to achieve a current appropriate for a paper type, a use environment, and other factors (see, for example, Japanese Patent Application Laid-open No. 2002-351234).
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-351234 applies constant currents having different values to a transfer roller during times in which paper is not being fed. The image forming apparatus then measures voltages when the respective currents are applied to thereby calculate a current-voltage characteristic of the transfer roller.
The different current values are set close to a minimum value and a maximum value of the current that flows through the transfer roller during paper feeding.
Thus, the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-351234 unfortunately entails occurrence of an electric discharge for the following reason. Specifically, when a load increases as a result, for example, of deterioration over time of the transfer roller, the application of the current close to its maximum value causes an excessively high voltage to be output also at the time of application of the current for estimation before the start of paper feeding.
Therefore, it is desirable to provide an image forming apparatus that does not develop a faulty symptom over time during application of a current.